Random Variables
by Starwind77
Summary: Their relationship was full of probabilities, but no matter how Charlie looked at them, it all came down to one value. Don x Charlie. Implied incest.


**Title:** Random Variables  
**Rating:** PG  
**Pairing:** Don x Charlie  
**Word Count:** 1,204  
**Description:** Their relationship was full of probabilities, but no matter how Charlie looked at them, it all came down to one value.  
**Author's Note:** Best application of my probability theory course, ever XD. Of course, I took artistic liberty with the mathematical metaphors, so you'll have to forgive bastardization of certain terms. Just think - it's all in the name of true sibling love!

Wanting Don was a Bernoulli random variable. Two discrete options - right or wrong, success or failure. Zero sum. He feels the chalk stutter between his fingertips at the other's approach, the numbers dance briefly out of focus when Don drops an arm around his shoulder, tousles his hair briefly or scowls in concern at the dark circles under his eyes, the case getting to him in more ways than one. What were the odds? Sometimes, he thinks hopefully, 1 to 100 on the days the beer flows freely and his brother's hand tangles just a bit longer in the wet curls of his hair. Other times, it's a long shot, 1 in a billion at best with the outliers removed, as he guiltily does the math in his head he'd never think to put to paper, gaze fixed on the silhouette of Don unfurling against the bedroom curtains. But mostly, he feels the probabilities slipping away under his brother's dark, sable eyes, telling Don he loved, he loved them, he loved them together more than any words or equations or cryptographic algorithms could describe.

It wasn't always like this.

Working with Don was a Gaussian distribution, a bell curve of perfectly symmetrical probabilities centered around a single peak. Or so Charlie liked to believe as he rushed into the FBI headquarters for the third time that day, laptop at the ready and metaphors not far behind, a half-eaten muffin balanced on top of his knapsack. The normality of randomness in this maze was only measurable by the amount of chaos controlled by his brother, which, according to the strain on Don's face, had rapidly approached asymptotic decay. He finds himself counting the number of lines that crease the other's brow, and computing their topology as they one by one smoothed out with the unraveling puzzle, abstract mathematics turning into hard facts, understanding. The light that illuminates his brother's eyes mirrors his own and sends a prickle down the nape of his neck.

"Good job, Charlie," Don tossed out in passing with a ready pat on the back, leaving him to wonder whether the spark he'd felt was an anomaly or just another point on the bell curve.

Arguing with Don was a bimodal distribution. Twin peaks on opposite ends, no middle ground, probabilities divided according to allegiance. He feels the hot flush of anger mix with frustration on his face, and ducks his head into another equation to avoid the remonstration he knows he'll receive. The staccato clacking of chalk intermingles with bridled rancor.

He couldn't explain the stitch in his chest, nor why a simple 'No' gets caught in his throat when he meets his brother's gaze.

"I need – I need to follow this algorithmic approach on my Markov matrices – " The numbers whirr madly in his head, drove him feverishly away like the painful proximity of his brother.

"Charlie, people are dying. Do you understand that? A girl died today, and the letter promised more to come."

"A doubly stochastic matrix, modeling the probability of ergometric pulse..." He couldn't take his eyes away.

"Listen to me, Charlie!"

" – with the convergence here, so if you would _kindly_ let me follow through on this line of thinking, then I could determine the center, the center limit of this function!"

A sharp clatter, and he finds his fingers trembling against the blackboard surface. Empty. Jagged mark across smooth green. The broken chalk rolls slowly like a bullet casing to his brother's feet.

"Oh, yeah?" Don finally says, quiet. "Well, maybe you should start looking for your center at home."

The numbers stop suddenly then, and like two modes twining into one, disappeared.

Apologies from Don were a Poisson process, and possibly poisonous too when the occasion arose. Charlie thinks that the rarity of the event is only superseded by their memorylessness over the years, repetition upon repetition, building one upon the other. From the first sibling scuffle at the koi pond to P vs. NP that night (he can't help but see his brother's name in P and everything else, an NP problem), they were never more predictably spaced. Only now, with each remorseful word – that trip in his heart at the slur in Don's voice, more feeling than eloquence, and the crash thereafter as he replies – the stakes had risen beyond an innocent step function into a permanent lump at the base of his throat.

Don opens his mouth because he's always the one to take responsibility first, but it's Charlie who places a palm to his chest and speaks.

"I'm sorry," he says simply.

He couldn't depend on them never remembering forever.

Confessing to Don was a Cauchy distribution. Charlie knew, because it was unknowable. An undefined mean, incalculable variance, all his carefully ordered probabilities contradicting themselves – _proven_ to contradict themselves – and rendering his predictions false, human end to a human problem that mathematics couldn't solve. If only positives were negatives, he thinks absently as he licks dry lips. But, no. Even the numbers deserted him. Only this moment stretching into infinity remained.

"Look, buddy...you gotta tell me what's wrong." They'd been sitting there for nearly an hour in silence. "Whatever it is, whoever it is, it's eating you up. You hear me? You can't keep doing this to yourself!" Hiding in math, Don means, but right now that couldn't be further from the truth.

His mind unbearably empty, Charlie leans on the edge of the sofa, unable even to compute the rotational speed of the chalk in his hand as a distraction from the task before him, let alone random variables. It was like –

"Resonance," he says.

"What?"

"Forced resonance. It's what the Cauchy distribution describes." He could see it, too, in the tremble of his voice, the flicker of Don's eyes, the way his heart couldn't stop from pounding in his throat. "You know, like a...like a violin string being plucked, the harmonics are endless, but they all – they depend on this one _fundamental_ frequency, and I can't..." He presses a palm to his head, as if it would make the metaphor clearer. "...I can't..."

Gently, Don catches his wrists, touches their foreheads together. "Charlie, what are you trying to say?"

"You, Don!" The outburst rings chaotically in his ears. "It's you, it's always been you, there couldn't ever be anyone but you, and when it came down to this..." Large brown eyes blink moist, as he whispers hoarsely. "I couldn't let go, you know? Of this." Softly, Charlie touches the side of his brother's cheek, willing with all his might for Don to understand. "This, the Eppes Convergence, was never meant to diverge."

There's something indescribably elegant about the way they glow in the lamplight, entwined together like binary stars, the luminescent shape of spectral lines shifting over lips and skin and hair.

Because loving Don...

"Charlie, are you thinking about math again?" His brother's sleepy voice murmured in his ear, mouth barely brushing warm skin, as a languid arm curled up to stroke the side of his face.

Loving Don was a constant, a universal ratio, no uncertainty or randomness at all.

"Nope." Charlie smiled sheepishly, and turned his head into the kiss. "Just thinking of you."


End file.
